Particular attention is being paid to a heterojunction bipolar transistor using compound semiconductor materials such as gallium arsenide and aluminum gallium arsenide because this attractive candidate for the next generation can provide a large current driving capability and a high frequency performance. In general, the switching speed of the heterojunction bipolar transistor is roughly dominated by three factors i.e., the accumulation time of the parasitic capacitance, the base transit time of minority carriers and the transit time across the depletion layer projecting from the base-collector junction into both base and collector regions, and each of the three factors occupies about one third of the total delay. Recent research and development efforts result in reduction of the parasitic capacitance and the parasitic resistance as well as reduction of the base transit time, but the third factor still remains unsolved.
In FIG. 1 of the drawings, there is shown a typical example of the energy band diagram produced in a heterojunction bipolar transistor which largely comprises an emitter region of an n-type aluminum gallium arsenide represented by the molecular formula of Al.sub.0.3 Ga.sub.0.7 As, a base region of a p-type aluminum gallium arsenide represented by the molecular formula of Al.sub.x Ga.sub.1-x As where x ranges between 0.1 and 0.0, and a collector region consists of a base junction section of a lightly doped n-type gallium arsenide ( n.sup.- GaAs ) and a collector contact section of a heavily doped n-type gallium arsenide ( n.sup.+ GaAs ). In the energy band diagram, the lower edge of the conduction band, the Fermi level and the upper edge of the valence band are abbreviated as Ec, Ef and Ev, respectively. The base junction section of the collector region is small in impurity concentration, so that a depletion layer deeply penetrates into the collector region. A strong electric field takes place across the depletion layer due to difference in potential between the base region and the collector region, and electrons reaching the depletion layer are subjected to the strong electric field, thereby immediately becoming hot electrons HE. The hot electrons are moved from the .GAMMA.-valley 1 to the L-valley 2, and the effective mass of each hot electron becomes large. Since most of the hot electrons are moved to the L-valley 2, the drift mobility is affected by the large effective mass and, for this reason, a large amount of time is consumed to terminate the traveling across the collector region. The transit time across the depletion layer mainly dominates the switching speed of the heterojunction bipolar transistor in the circumstances described above, so that a solution is needed to improve the heterojunction bipolar transistor.
FIG. 2 shows an energy band diagram produced in another heterojunction bipolar transistor comprising an n-type collector region, a p-type base region and an n-type emitter region having a wider bandgap than the base region. In this prior-art heterojunction bipolar transistor, a depletion layer takes place and deeply penetrates into the collector region, and a strong electric field is produced across the depletion layer due to difference 3 in potential between the base region and the collector region. Then, electrons are also subjected to the strong electric field and becomes hot electrons HE. These hot electrons are moved into the L-valley as similar to those in the heterojunction bipolar transistor shown in FIG. 1, so that the drift mobility is decreased.
One of the attempts to reduce the transit time across the depletion layer is to absorb the potential difference between the base region and the collector region by using a p-n junction formed by a heavily doped p-type region and a heavily doped n-type region. FIG. 3 shows an energy band diagram produced in a heterojunction bipolar transistor of the type having a p-n junction formed by a heavily doped p-type region and a heavily doped n-type region. The heterojunction bipolar transistor comprises an emitter region of an n-type aluminum gallium arsenide represented by the molecular formula of Al.sub.0.3 Ga.sub.0.7 As, a base region of a p.sup.+ type aluminum gallium arsenide represented by the molecular formula of Al.sub.x Ga.sub.1-x As where x ranges between 0.1 to 0.0, and a collector region consisting of three sections. The collector region is provided with the three sections respectively formed of an intrinsic gallium arsenide ( i-GaAs ), a heavily doped p.sup.+ type gallium arsenide and a heavily doped n.sup.+ type gallium arsenide, and the second section of p+type gallium arsenide is extremely thin. In this prior-art heterojunction bipolar transistor, a difference in potential between the base region and the collector region is absorbed by the p-n junction formed by the second and third sections, so that a relatively small electric field takes place across the first section of the intrinsic gallium arsenide. For this reason, electrons mostly travel along the .GAMMA.-valley without moving into the L-valley over the first section of the intrinsic gallium arsenide corresponding to the depletion layer produced in the collector region shown in FIG. 1. If the electrons travel along the .GAMMA.-valley, each electron is small in effective mass, so that a delay is hardly introduced in the transit time across the first section of the intrinsic gallium arsenide.
This solution is also employed for the heterojunction bipolar transistor shown in FIG. 2. An energy band diagram produced in the heterojunction bipolar transistor of the type having a p-n junction formed by heavily doped sections is illustrated in FIG. 4, and most of the potential difference 4 between the base region and the collector region is absorbed by the potential difference 5 formed by the heavily doped sections. However no further description is hereinunder incorporated so as to avoid a repeat.
However, a problem is encountered in the prior-art heterojunction bipolar transistors illustrated in FIGS. 3 and 4 in stability under operation. Namely, each collector has the second and third sections heavily doped with the p-type impurity atoms and the n-type impurity atoms, respectively, so that the p-n junction formed therebetween is liable to be broken down with a relatively small reverse bias voltage. This breakdown results in unstable operation. Moreover, a relatively small potential barrier for holes takes place between the base region and the collector region, so that holes HL in the base region tend to be forwarded to the collector region under a high injected condition as shown in FIG. 3. This also results in unstable operation. Of course, the stability in operation may be controlled by selection of biasing conditions, however this solution is not recommendable because of small biasing tolerance.
Another problem encountered in the prior-art heterojunction bipolar transistors is formation of the second section of the heavily doped p-type gallium arsenide. The second section is extremely thin and heavily doped with beryllium atoms, so that the beryllium atoms tend to diffuse from the thin second section during high temperature processes after the doping step. If the beryllium atoms are diffused from the thin second section, the second section is decreased in impurity atom concentration. The high speed operation is realized in so far as the thin second section keeps the high impurity atom concentration. This means that each heterojunction bipolar transistor can not realize the high speed operation.